Exporting Watermark in Aperture 2.1

I am exporting pictures as a jpg at level 12 quality. The pictures have been cropped as 4 x 6" size images. I have created a watermark file in photoshop and saved as a psd file. When I export the 4 images the watermark is a different size on each of the 4 images. I have also tried clicking the scale watermark checkbox and all have a different size watermark. Any suggestions. Thanks